Hoist the iwc import in refresh_iwc_manifest to module top

The local import claimed to keep cachetools/requests out of celery's
startup graph, but cachetools is already pulled in transitively
(NotificationManager -> sse_dispatch) and requests is resident in any
worker that boots galaxy.app. There's no circular-import risk either, and
the sibling refresh_gtn_database imports GTNSearchDB at the top -- so hoist
this one for consistency and drop the comment.

lib/galaxy/celery/tasks.py

@@ -26,6 +26,7 @@ from sqlalchemy import (
 )
 
 from galaxy import model
+from galaxy.agents import iwc
 from galaxy.agents.gtn import GTNSearchDB
 from galaxy.celery import (
     celery_app,
@@ -840,11 +841,6 @@ def refresh_iwc_manifest(config: GalaxyAppConfiguration):
     so an iwc.galaxyproject.org outage doesn't kill the periodic queue --
     on-demand callers still get the prior cached copy until the TTL lapses.
     """
-    # Local import keeps the iwc module (and its cachetools / requests
-    # imports) out of celery's startup graph -- they only load on the
-    # workers that actually run this task.
-    from galaxy.agents import iwc
-
     try:
         manifest = iwc.refresh_manifest()
     except Exception as e:  # noqa: BLE001 -- best-effort warm; resilience over precision